This invention relates to improvements in line protectors of the type used for protecting telephone lines and other communication lines from over-voltage conditions.
More particularly, the present invention is concerned with protectors of the type intended for protection of wire conductors and equipment connected thereto from electrical over-voltage and over-current conditions which may result from electrical power surges, lightning, and the like. One such protector known in the art includes an arrester comprising a cold cathode gas discharge tube commonly called a gas tube. A gas tube arrester generally comprises a pair of electrodes spaced apart by an insulator to form an arc gap therebetween, and sealed so that the arc gap may be filled with a gas, e.g. argon gas. It is known in protectors of this type to provide a "fail-safe" operation, that is, the protector provides a direct metallic current path from the line to ground in the event of an over-current condition which would tend to damage the gas tube arrester. However, gas tube arresters of this type also tend to fail in an open or high voltage condition. By this is meant that the gas tube fails to discharge at its rated voltage so as to ground the line in response to an over-voltage thereon at which the arrester is designed to operate. Such a failure is usually due to leakage due to a broken seal or similar damage to the gas tube arrester which allows the gas to escape therefrom. It will be appreciated that a gas tube arrester which has failed in this fashion is difficult to detect, as the line to which it is connected continues to operate properly. It will further be appreciated that it is most difficult and expensive to check individual gas tube arresters for a fail open condition, since such arresters are commonly used in large numbers and at a plurality of locations.
Thus, it is desirable to provide an improved gas tube arrester-type protector including means for providing "back up" line protection in the event the gas tube arrester fails open. One such arrangement known in the prior art and shown in Menninga U.S. Pat. No. 4,002,952, issued Jan. 11, 1977, comprises a protector including a gas tube-type arrester electrically connected in parallel with a carbon air gap-type arrester. In this arrangement, then, the carbon air gap arrester tends to continue to provide line protection over a relatively high number of over-voltages or the like, after failure of the gas tube arrester. In contrast, it is desirable to provide a secondary arrester or the like, adapted to fail to ground fairly rapidly upon failure of the gas tube arrester, and preferably upon the first over-voltage condition on the associated line following such failure. It will be appreciated that such a rapid failure to ground of the secondary arrester provides a prompt and positive indication of the failure of the gas tube arrester, whereupon the gas tube arrester may be promptly replaced. The prior art arrangement also suffers from the disadvantage that a specially designed, non-standard type of gas tube arrester is required therein, to fit, together with the carbon air gap arrester, in a standard protector receptacle, thus increasing the expense of providing such a protector unit.